Unilateral hearing loss completely eliminates localization ability that depends on interaural time and level differences

What it is

People with only one functioning ear (single-sided deafness) don't experience "somewhat worse" localization — they lose entirely the ability to judge direction by comparing signals between the two ears. That comparison-based localization mechanism needs two functioning ears participating at once; with one missing, the comparison itself simply cannot happen — there is no partial, "discounted" middle state.

It's easy to imagine this scaling proportionally with the severity of bilateral hearing loss. Functionally, though, complete deafness in one ear and moderate loss in both ears are different kinds of problems: in the latter, both ears still supply a signal, just at reduced precision; in the former, half of the input the comparison needs is simply gone — a difference in kind, not degree.

Why it happens

Binaural localization works by comparing the difference between the signals arriving at the two ears; once only one ear can supply a usable input, that comparison ceases to exist mathematically — the same underlying situation as an audio signal collapsed to mono being unlocalizable, except here it's the listener's own sensory system, not the sound signal, that has lost the input.

People with unilateral hearing loss try to partially compensate through alternative means — turning the head, or using the head shadow effect (the head acts like a baffle, attenuating sound before it reaches the one working ear when the source is on the far side) to roughly judge left versus right — but these compensations are far less precise and far slower than simultaneous binaural comparison, and they cannot resolve front-back discrimination at all. The head shadow effect also brings an additional practical consequence: if a noise source happens to sit on the hearing-ear side while the target speech is on the deaf-ear side, the hearing ear receives both a loud noise and a target signal already weakened by head-shadow attenuation, giving a much worse effective signal-to-noise ratio than with two normal ears. Conversely, if the noise is on the deaf-ear side and speech is on the hearing-ear side, the head shadow effect actually helps, reducing the noise's interference at the hearing ear. This means a unilateral-loss user's performance in noise depends heavily on where noise and target sound sit relative to the hearing ear, not on some fixed ability level.

Studying it

A common approach has participants with unilateral hearing loss perform standard direction-localization tasks, comparing error magnitude and confusion patterns against a bilaterally normal-hearing control group; studies of the head shadow effect systematically vary a noise source's position relative to the hearing and deaf ears, measuring how the speech reception threshold shifts with noise position to quantify the performance gap between "noise on the hearing-ear side" and "noise on the deaf-ear side."

Common independent variables: source direction, noise position relative to the hearing/deaf ear, whether the loss is congenital or acquired (which may affect the degree of compensation). Common dependent variables: direction judgment error, magnitude of speech reception threshold shift with noise position.

Where it stops holding

• A cochlear implant or a contralateral routing of signal (CROS) hearing aid can route sound from the deaf side to the hearing ear, improving audibility (whether the sound can be heard at all), but this is signal routing rather than true restoration of binaural comparison — spatial information is compressed or lost in the process and should not be equated with restored localization ability.
• Congenital and acquired unilateral hearing loss may differ in degree of compensation; congenital cases often show more thorough lifelong adaptation to the remaining monaural cues (head shadow, pinna spectral cues), so compensation varies by individual and shouldn't be summarized with a single magnitude across the whole unilateral-loss population.
• This entry addresses loss of horizontal-plane localization; front-back and up-down direction inherently rely on additional monaural cues (see the discussion of binaural localization's boundaries in the related group), and the impact of unilateral loss on those directions isn't identical to its impact on the horizontal plane.

Applying it

• Don't treat stereo or spatial audio directional cues as the sole channel for conveying critical directional information — if "the alert is coming from your left" is conveyed only through channel differences, a unilateral-loss user cannot receive it at all; pair it with a visual directional indicator or a lateralized haptic cue as a redundant channel.
• When testing voice call or conferencing features' performance in noise, specifically set up and validate the "noise on the hearing-ear side" scenario — this is the worst case, where the head shadow effect provides no help and unilateral-loss users perform worst, rather than testing only symmetric noise conditions.
• When evaluating accessibility, simulate unilateral hearing loss with real single-ear occlusion (rather than simply lowering the volume on one side overall), to accurately reproduce the combined localization and speech-in-noise difficulties these users actually face.

Related

• Same group: A3.17.1 Conductive hearing loss affects loudness perception; sensorineural loss also degrades frequency resolution · A3.17.3 Compression in hearing devices alters the intended loudness hierarchy of digital alert sounds · A3.17.4 A normal audiogram with difficulty understanding speech in noise is an easily overlooked hearing impairment
• Nearby: A3.05 Sound source localization · A3.09 Environmental noise and signal-to-noise ratio
• Search terms: single-sided deafness · head shadow effect · CROS hearing aid · binaural hearing loss